Carbon group elements
The elements carbon, silicon, germanium, tin and lead constitute the 14th group of the periodic table. These are p-block elements having the configuration ns2np2.
Element At.No. Electronic structure
Carbon 6 [He] 2s2 2p2
Silicon 14 [Ne] 3s2 3p2
Germanium 32 [Ar] 3d10 4s2 4p2
Tin 50 [Kr] 4d10 5s2 5p2
Lead 82 [Xe] 4f14 5d10 6s2 6p2
Allotropic forms of carbon
Carbon exhibits allotropy and occurs as
1. Diamond, a beautiful crystalline substance
2. Graphite, a soft greyish black crystalline substance
Amorphous carbon, black residue left when carbon compounds are heated.
Different amorphous varieties of carbon are
(iv) Bone black or animal charcoal,
(vi) carbon black,
(viii) Gas carbon and
(ix) petroleum coke.
Structure of diamond
In diamond every atom is bonded with the other by covalent links resulting in the formation of giant molecule. Each carbon atom is linked with four neighbouring carbon atoms held at the corners of a regular tetrahedron by covalent bonds. The C-C bonds are very strong. The crystal of diamond is very hard and has high melting and boiling points.
The combined strength of the many carbon-carbon bonds within the structure of diamond give it both great hardness and a lack of chemical reactivity.
Structure of graphite
It consists of separate layers. The carbon atoms are arranged in regular hexagons in flat parallel layers. There is no strong bonding between different layers, which are, therefore, easily separable from each other. Since there are no covalent linkages between the adjacent planes, graphite can be easily cleaves along the lines of the planes. Whilst the bonds within the layers are strong, those between the layers are not and so they slide over each other easily This accounts for the softness and lubricating power of graphite.
Structure of Buckminster fullerenes
In 1985, a new allotrope of carbon was discovered by Richard Smalley and Robert Curl of Rice University, Texas, working with Harry Kroto of Sussex University. The first to be identified and the most symmetrical of the family, with 60 atoms and 32 sides (20 hexagons and 12 pentagons), was nick named `buckyball' and was then named buck minister fullerene, because it resembles the geodesic domes developed by an American inventor called R.Buckminister fuller. The group of spherical carbon molecules is called fullerenes. These compounds have superconducting properties and its potential for opening new areas of chemistry have made study of the `buckyball' as one of the most rapidly expanding areas of chemical research.
Amorphous form of carbon
Amorphous carbon is the most reactive form of carbon. It burns relatively easily in air, thereby serving as a fuel, and is attacked by strong oxidising agents. This form has structural features of graphite, such as sheets and layers. It's atomic structure is much more irregular.
Carbon and silicon are non-metals, germanium is a metalloid while tin and lead are metals. Thus metallic character increases on descending the group since ionization energy decreases on descending the group.
All of these elements form covalent hydrides though the number of hydrides and the ease with which these are formed decreases from carbon to lead. Carbon gives a vast number of hydrides (alkanes), silicon and germanium (silanes and germanes) whereas stannane (SnH4) and plumbane (PbH4) are the only hydrides of tin and lead are known.
Unlike alkanes, silanes are strong reducing agents, explode in chlorine and are readily hydrolysed by alkaline solutions. The difference is probably due to the difference in electronegativity between C and Si resulting in difference between C-H and Si-H linkages.
All these elements give tetrahalides. Tetrachlorides are usually fuming liquids at ordinary temperature. Carbon tetrahalide resists hydrolysis. This is because due to the absence of d-orbitals. Maximum covalency of carbon is only four and there is no possibility of formation of coordinate linkages with H2O, which could lead to hydrolysis.
Tetrahalides of rest of the elements undergo hydrolysis. For example
SiX4 + 2H2O SiO2 + 4HX
Carbon, silicon and germanium form trihalides of the type MHX3. Lead and tin do not form trihalides. Silicon, germanium, tin and lead form dihalides.
1. The chlorides are all simple molecular substances with tetrahedral molecules.
2. The stability of the chlorides decreases down the group and the +2 oxidation state becomes more stable than the +4 state. Only tin and lead form chlorides in which their oxidation state is +2, the other chlorides existing solely in the +4 state. Tin(II) chloride is a solid that is soluble in water, giving a solution which conducts electricity. It is also soluble in organic solvents. Its melting point is 246deg C. Lead(II) chloride is also a solid. It is sparingly soluble in water. The chlorides of the group 14 elements in their +4 oxidation state illustrate further the change in character of the elements from non-metal to metal down the group and giving a solution which conducts electricity, and melts at 501deg C. These observations suggest that tin(II) chloride has both covalent and ionic character, while lead(II) chloride is predominantly ionic.
3. All the chlorides with +4 oxidation state are readily hydrolysed by water, except tetrachloromethane (CCl4).
Compounds of carbon with less electronegative elements (eg. metals, Be, B, Si etc.) are called carbides. These are of three main types.
i. Ionic or salt-like eg. acetylides, methanides, allylides
ii. Interstitial or metallic eg. WC and
iii. Covalent eg. B4C, SiC.
All the three types of carbides are prepared by heating the element or its oxide with carbon or a hydrocarbon to a high temperature.
2Be + C -- > Be2C
CaO + 3C -- > CaC2 + CO
SiO2 + 3C -- > SiC + 2CO
1. The oxides show a marked trend in structure from the molecules of carbondioxide to giant structures intermediate between ionic and covalent lower down the group.
2. The +2 oxidation state is the more stable state in the case of leadoxide, and lead (IV) oxide decomposes on heating giving lead(II) oxide, a solid that melts at 886degC. The structure of lead(II) oxide is predominantly ionic.
3. The oxides at the top of the group (CO2 and SiO2) have an acidic nature, the carbonate ion CO32- being produced easily in dilute aqueous solutions. The ease of formation of oxoanions (SiO32-, GeO32-etc.) decreases down the group as the acidic character decreases. The oxides of germanium, tin and lead are amphoteric, reacting to form simple salts with acids.
Uses of carbon and its compounds
1. Carbon and its compounds play an enormous role in the global economy, eg. Fossil fuels.
2. Halogenated carbon compounds are used as refrigerants, aerosol propellants, fire extinguisher and solvents.
3. CS2 is used in the manufacture of viscose rayon (artificial silk) and cellophane.
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